ABSTRACT - PROJECT 1 Based on discoveries made in the previous cycle of our program project, Project 1 will probe deeply into the cellular and molecular events of CCM pathogenesis. Using lineage tracing of murine CCM lesions we showed that the initial event of lesion genesis is somatic loss of the remaining wild-type copy of the Ccm gene, followed by a clonal expansion of endothelial cells harboring the same somatic mutation, and what appears to be recruitment of additional ECs into the growing lesion ? cells that do not harbor the somatic mutation. Secondly, based on discoveries in the Kahn lab for a role for activated PI3 kinase in lesion development in mouse CCM models, we identified activating PIK3CA mutations in more than 50% of human CCM lesions. These collaborative discoveries suggest we have not yet fully understood the molecular and cellular events that contribute to CCM lesion genesis, growth and maturation. In the first Aim, we will deeply sequence bulk human CCMs to determine whether somatic mutation of any other oncogenes contribute to the development of lesions. In a second Aim we will use single-cell genomic DNA sequencing to determine whether individual endothelial cells require both the somatic CCM mutation and the PIK3CA mutation (and/or mutation of other genes discovered in Aim 1), or instead whether these genes are mutated in separate endothelial cell compartments of the lesion. In Aim 3, due to growing evidence that the somatically mutated cells poison the non-mutant cells to recruit them into the growing CCM, we will employ single-cell RNA sequencing to determine how the somatic mutation profile of the lesional cell influences the cell?s gene expression profile. In addition to increasing fundamental understanding of CCM pathogenesis, this work has therapeutic implications. The PI3K-AKT signaling pathway is a target of existing drugs with others under development. Thus as part of this project we will continue our quest for a highly effective therapy by testing compounds nominated in this or any of the three projects, using our more clinically-relevant CCM mouse models developed in the past funding cycle. With a more complete knowledge of the molecular and mutational signature of CCM lesional cells, we hope to identify new targets for drug repurposing for CCM patients.